Industrial equipment may be coupled to a communications bus. Using the communications bus, the industrial equipment can communicate with other industrial equipment or devices coupled to the communications bus. For example, a flow measurement and a flow control device may be coupled to a common pipeline that carries a material. The flow measurement and flow control device may communicate with each other over the communications bus to control a property of the material. More specifically, the flow measurement device may measure the properties of the material and provide the measurements to the communications bus. The flow control device may control the properties of the material using the measurements obtained from the communications bus.
The industrial equipment frequently employ, or are exposed to, high voltages. In addition, the communications bus typically include electrical conductors. As a result, there is potential that the high voltages will electrically couple to the communications bus. This can damage the other equipment or devices coupled to the communications bus and cause catastrophic events. Therefore, many safety regulations require that the communications bus be electrically isolated from the industrial equipment. The safety regulation may also specify a minimum isolation voltage rating for the electrical isolation.
Optocouplers are used to isolate the communications bus from the industrial equipment. The optocouplers employ an isolation barrier. The isolation barrier may be a transparent dielectric with a maximum isolation voltage rating, which may be greater than the minimum isolation voltage rating specified by the safety regulation. A light emitting diode (LED) is on one side of the isolation barrier and a photodiode is on the other side of the isolation barrier. The LED may be coupled to, for example, a controller that is in communication with the industrial equipment and the photodiode may be coupled to the communications bus. The LED side of the isolation barrier is commonly referred to as the non-intrinsically safe (non-IS) portion and the photodiode side of the isolation barrier is commonly referred to as the intrinsically safe (IS) portion.
The optocouplers are prone to issues that adversely affect a relationship between an input and an output of the optocoupler. For example, the output from the optocoupler may not be linearly related to a voltage or current applied to the optocoupler's input. The non-linearity may be due to the LED's sensitivity to temperature, a drift in the LED's brightness, etc.
High-linearity optocouplers may address these issues. The high-linearity optocouplers may include a feedback photodiode in the non-IS portion. An external amplifier can be used with the feedback photodiode to monitor the light output of the LED and automatically adjust the LED current to compensate for any non-linearities or changes in the light output of the LED. The feedback amplifier acts to stabilize and linearize the light output of the LED.
FIG. 1 shows an exemplary optocoupler 20, which may be a high-linearity optocoupler, that is communicatively coupled to an optodriver circuit 10. The optodriver circuit 10 receives a signal at an input IN. The signal can be in any appropriate form, but for the sake of discussion, the signal received at the input IN is a pulse width modulation (PWM) signal. The optodriver circuit 10 may convert the PWM signal to an analog signal that may be proportional to the PWM signal. The optodriver circuit 10 may also filter and amplify the analog signal and provide the filtered and amplified analog signal (“conditioned signal”) to the optocoupler 20.
As shown in FIG. 1, the conditioned signal is provided to an LED 22. The LED 22 emits light in response to the conditioned signal. A first photodiode 24 and a second photodiode 26 receive the emitted light. An isolation barrier 28 is between the LED 22 and the second photodiode 26. Therefore, the output OUT of the optocoupler 20 is electrically isolated from the input IN. The first photodiode 24 provides a feedback signal to the amplifier circuit 12. The amplifier circuit 12 receives the feedback signal and adjusts the conditioned signal. Accordingly, the light emitted from the LED 22 may be controlled to ensure that the light emitted by the LED 22 is proportional to the signal received by the amplifier circuit 12. However, the output OUT does not have a linear relationship with the input IN, even if the optocoupler 20 is employed, as is explained in more detail in the following.
The safety regulations also typically limit a voltage that may be applied to the LED 22 and the first photodiode 24 to a maximum voltage. Voltage limiters 14 and 16 shown in FIG. 2 may limit the voltage applied to the LED 22 and the first photodiode 24. The voltage limiters 14, 16 are configured to conduct current to ground if the voltage applied to the LED 22 or the first photodiode 24 is greater than a breakdown voltage of the voltage limiters 14 and 16. The breakdown voltage of the voltage limiters 14 and 16 may be less than the maximum voltage specified by the safety regulation.
A typical configuration of the voltage limiters 14 and 16 are shown in FIG. 3. In the typical configuration, the voltage limiters 14 and 16 are Zener diodes D1, D2, D3. The Zener diodes D1, D2, D3 are electrically coupled to the LED 22 and the first photodiode 24. As can also be seen, the optodriver circuit 10 also includes resistors R1, R2, R3 and fuses F1, F2, F3 that protect the Zener diodes D1, D2, D3. A low pass filter (LPF) filters the signal received at the input IN.
The Zener diodes D1, D2, D3 are not perfect insulators below the breakdown voltage. That is, the Zener diodes D1, D2, D3 may have a leakage current, which can vary from diode to diode, at an operating voltage. For example, the first Zener diode D1 may have a leakage current that is less than the leakage current through the third Zener diode D3. This can cause the conditioned signal provided to the LED 22 to vary. As a result, the output OUT does not have a linear relationship to the input IN.
In other words, the relationship between the input IN and output OUT is not linear due to the configuration of the Zener diodes D1, D2, D3 shown in FIG. 3. Accordingly, there is a need for an input protection circuit for an analog optocoupler and, in particular, an input protection circuit that does not cause variations in the conditioned signal provided to the optocoupler.